Multiplexing, Switching and Routing -...

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Multiplexing��”From one channel to multiple channels”

How to share one medium while facilitating multiple channels of communication:

Frequency Division and Time Division Multiplexing


Frequency Division Multiplexing (FDM alla FSK)

Σ subcarrier fsc1

subcarrier fsc2

demodulator fsc1

band pass filter fscn

band pass filter fsc2

band pass filter fsc1

Receiver fc

Transmitter fc

subcarrier fscn

demodulator fsc2

demodulator fscn


Standards # voice channel

bandwidth spectrum US/AT&T ITU_T

12 48 kHz 60-108 kHz Group Group

60 240 kHz 312-552 kHz Super Group Super Group

300 1,23 MHz 812-2044 kHz Super Group Master Group

600 2,52 MHz 564-3084 kHz Master Group Master Group

900 3,87 MHz 8,52-12.39 MHz Master Group Super Master Group

3600 16,98 MHz 0.56-17,55 MHz Jumbo Group Jumbo Group

10800 57,44 MHz 3,12-60,57 MHz Jumbo Group Multiplexed

Jumbo Group Multiplexed

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Example: ADSL ADSL Asymmetric Digital Subscriber Line

Originally for Video-on-Demand: less control going up – lots of image going down

è Very similar to internet usage !!!!!!!

Multiple “regular” phone connections at the same time on which QAM (Quadratic Amplitude Modulation) is implemented

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•  Reserve lowest 25 kHz for Voice (POTS, Plain Old Telephone Service) 25 instead of 5 to prevent cross talk between voice&data

•  Facilitate two bands: small upstream / big downstream

•  Use FDM within upstream and downstream band

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ADSL 4 kHz channels

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ADSL standards

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Time Division Multiplexing(TDM)

Buffer

Buffer Modem

Buffer

Buffer

Buffer

Buffer

1 2 n n1 2 …………… n 1 2…………… ……………

channel

Frame

Synchronous TDM: not synchronous but frames are fixed and slots are always filled


How is framing implemented

0101 ….. control channel

Added digit framing

Pulse Stuffing

Frequency > Σ (freq. of the sources)

additional bits are added at fixed position in the frame


Relationship with data link framing

F1f2A1F2C1A2d1C2d1d2.................

for

F1f1f1d1d1d1C1A1F1 in characters flag control

information FCS

address


Example: Telephony DS-1 transmission format

Voice è PCM (8000 samples per second, 8-bit) TDM-frame = 24 (channels) x 8 bits +1 (frame bit) = 193 bits Data rate: 8000 x 193 = 1.544 Mbps DATA only 23 out of 24 channels used, 24th channel has special SYNC BYTE per channel 1 bit for user/system data

è 7 x 23 x 8000 = 56 x 23 kbps = 56 kbps p. channel


Standards Telephony US/JAPAN ITU-T

# channels Mbps Level #channels Mbps

DS-1 24 1.544 1 30 2.048

DS-1C 48 3.152 2 120 8.448

DS-2 96 6.312 3 480 34.368

DS-3 672 44.736 4 1920 139.264

DS-4 4032 274.176 5 7680 565.148

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Network SWITCHING Next to multiplexing: switching is required to realize multi to multi connections Especially needed in Wide Area Networks (WAN) Also present in Local Area Networks (LAN) or in multi processors architectures.


Circuit Switching

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1.  A dedicated path between two end stations is realized or channel (TDM/FDM)

2.  Data is being transmitted (Switches don’t inspect data) 3.  Path is broken up


Circuit switching

IMPORTANT CHARACTERISTIC:

BLOCKING VS NON-BLOCKING

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Connection cannot be realized because all paths are occupied

Connection can always be realized and at any time


Space Division Switching: ��non-blocking Crossbar Switch

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Very costly: N2 switches


Multistage Networks Use many small crossbar switches and connect them wisely.

Blocking can occur!!!!!

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8

6

3

1

5 cannot be connected to 2


Omega Networks��(based on Perfect Shuffles)

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2logN + 1 stages: non blocking with O(NlogN switches)


Variants of PS networks��Cube Network

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Butterfly Network

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Fat Tree Network

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Time Division Switching Do not confuse with TDM !!!

Bus

Bandwidth of Bus > Σ indiv. bandw.

Then non-blocking!


Routing in Circuit Switched Networks

•  Alternate Routing è Each switching node has its own routing table •  Fixed Alternate Routing

Routing tables do not change •  Dynamic Alternate Routing

Depending on time (e.g. time of the day) routing tables will change

•  Adaptive Routing Central Controller gets status of all switches and gives routing updates to all switches

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First choice Second choice

A to B Via switch i Via switch j

A to C Via switch j Via switch k


Packet Switching

•  Datagram Every packet is routed independently è As a consequence packets can arrive out of order

•  Virtual Circuit (Wormhole) Before communication is initiated a Call-Request packet is sent on the network, which fixates a virtual path between sender and receiver. è Packets arrive in order, but it not as flexible as datagram

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Data is sent by packets (usually < 1000 octets), Every switching nodes has buffers


Summarizing

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Circuit Switch Virtual Circuit Datagram

ACK signal

Call request

Call accept


Different Combinations

•  External Virtual Circuit & Internal Virtual Circuit

•  External Virtual Circuit & Internal Datagram

•  External Datagram & Internal Virtual Circuit

•  External Datagram & Internal Datagram

Which one makes sense?

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Routing Trade Offs

Correctness Simplicity

Robustness Stability

Fairness Optimality

Efficiency

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Routing for Packet Switched Networks

Like circuit switching can we differentiate between: Fixed Routing and Alternate Routing è No difference between datagram and virtual circuit

Random Routing Every node chooses randomly outgoing link, based on some prob. Distribution, e.g. Pi = Ri / ΣRj

with Ri data rate possible on link i.

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Flooding (very inefficient, very reliable) Every node puts incoming packet on every outgoing link, except the incoming link è Exponential growth 1. Every node logs all the packets If packets arrives a second time: discard 2. Every packet, contains counter: hop-count If hop-count > threshold: discard

Adaptive Routing (Central vs Distributed) Every node gets network status information Ø  Local, e.g. queue length of the outgoing links Ø  Adjacent nodes Ø  All nodes

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ARPANET��based on Adaptive Routing, Distr. & Adjacent Nodes

First Version: 1969 Based on Bellman-Ford Algorithm Every node i has two vectors:

Di = , Si = Every 128 ms every node exchanges delay vector with adjacent nodes. Then every node k: dkj = MiniεA [dnew

ij + dki ] and skj = i, the node i which minimizes dkj. Link delays are the queue length for that link.

Disadvantages: Link delays were not accurate Thrashing would occur

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di1 di2

diN

si1 si2

siN

N = #nodes dij = estimated delay from node i to j sij = next node on the route i to j


2de Generation (1979) Every node: Ø  Timestamp on incoming message (arrival time) Ø  Departure time recorded Ø  If pos. ACK is received: delay = (dept. time – arrival time)

Every 10 sec: every node computes the average delay per link If delay is different: FLOODING is used to inform all the other nodes Every node gets status of the whole network!!!!!!!!! Dijkstra’s shortest path algorithm is used to compute new routing table

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3de Generation (1987) When load is heavy: Observed delay under old routing ≠ delay under new routing è  Oscillation effects è  Instead of BEST route: a “good” route Smoothening of link costs (delays) Every 10 seconds: 1.  (Queuing theory) ρ = 2(s-t)/(s-2t), with

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ρ=link utilization t = measured delay s = service time

2.  U(n+1) = 0.5ρ(n+1) = 0.5U(n), U(n) average utilization 3.  New delays are computed based on U(n), terrestrial: 1 Hop for U(n)<0.5, 2 Hops for U(n)>0.8 sattelite: 2 Hops for U(n)<0.8

Otherwise the same as 2-de generation

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